Transformation toughened zirconia is associated with volume expansion which results from the tetragonal to monoclinic crystal phase transition. The transformation equilibrium is a function of several variables such as temperature, the presence of stabilizer additives, and grain size.
Due to the sensitivity of pure zirconia transformations to temperature changes, stabilizer additives have been incorporated into zirconia to suppress undesirable and uncontrollable tetragonal to monoclinic transformations. Such additives favor a particular phase, effectively arresting the transformation equilibrium at a point where the crystal phase exhibits properties most advantageous for a desired structure. Transformation toughened, tetragonal, partially-stabilized zirconia, and ceramic matrix materials toughened by tetragonal, partially-stabilized zirconia have been found useful where excellent thermal conductivity, hardness, and toughness properties are required; such as in wear/abrasion resistant ceramics, thermal shock resistant ceramics, cutting tools, draw dies, ceramic bearings, and oxygen ion conductors.
Heretofore, the focus of stabilizer additive use has been to add a particular oxide dopant to the zirconia to arrest the transformation equilibrium reaction at a particular stage of its development. As a result, exotic and costly additives at moderate concentrations such as yttria, ceria, and combinations of other rare earth oxides have been mixed with relatively rare zirconia to produce a material with highly desirable properties. For example:
U.S. Pat. No. 4,565,792 discloses a stabilized zirconia doped with yttria, stating that a minimum of 1.7 mole percent yttria is needed for stabilization. U.S. Pat. No. 4,520,114 discloses the utility of a 1-30 mole percent mixture of alkaline earth and yttrium metal oxide for necessary stabilization. U.S. Pat. No. 4,430,440 discloses an alumina, titania, yttria, zirconia composition; however, the small amount of zirconia present precludes its consideration as a zirconia based alloy system.
In the abstracts from the Zirconia 1986 meeting held in Tokyo, September, 1986, K. Tsukuma et al. "Transparent Titania-Yttria-Zirconia Ceramics", discuss the use of the single composition of 90 mole % (ZrO.sub.2 -8 mole % Y.sub.2 O.sub.3) / 10 mole % TiO.sub.2 for its optical properties. This composition has a cubic structure with grain sizes from 25 to 150 microns.
From this same meeting, C. Bateman et al. discuss in their abstract "Phase Equilibria and Phase Transformations in ZrO.sub.2 -TiO.sub.2 and ZrO.sub.2 -MgO-TiO.sub.2 Systems", four compositions; ZrO.sub.2 -15 mole % TiO.sub.2 ; ZrO.sub.2 -28 mole % TiO.sub.2 ; 95 mole % (ZrO.sub.2 -13 mole % MgO) / 5 mole % TiO.sub.2 and 80 mole % (ZrO.sub.2 -13 mole % MgO) / 20 mole % TiO.sub.2. Bateman et al. state that the two zirconia titania compositions were monoclinic at room temperature after sintering at 1400.degree. C. for one hour. They found that the zirconia-13 mole % magnesia-20 mole % titania sample was predominantly in cubic and monoclinic phases at room temperature with some MgTi.sub.2 O.sub.5 phase present. Nowhere was the substitution or addition of Y.sub.2 O.sub.3 discussed.
Herein is disclosed for purposes of initiating stabilization, a minimum of 0.25 mole percent yttria where zirconia is mixed with titania as a stabilizing aid. This novel mixture provides a transformation toughened material with attractive toughness and hardness properties, yet significantly decreases the required amount of yttria and zirconia.